1. Field of the Invention
The invention relates to a cast-iron piston ring for a piston of an internal combustion engine, the ring having a nitrided layer over its entire circumferential surface, with the nitrided layer comprising a diffusion layer and a connection layer disposed above the diffusion layer.
2. Description of the Related Art
Electrodeposited hard chromium layers or flame-sprayed molybdenum layers are used as anti-wear layers of piston ring surfaces, particularly the running surface, in series use of internal combustion engines. As the power of the engines is increased, these layers reach their individual limits of usefulness with respect to their anti-wear, anti-corrosion and fatigue-limit behavior. To improve the anti-wear behavior, piston rings were exposed to thermochemical treatment methods, among them nitriding. The components to be nitrided are heated in a gas, for example at temperatures between 450.degree.-600.degree. C., with the temperature being maintained for several hours. Nitrogen that has diffused into the surface forms a connection layer comprising a nitrided layer and a diffusion layer, in which the nitrogen content drops continuously until it reaches a residual quantity. The course of the nitrogen content over the cross section corresponds to the course of the hardness.
Because the connection zone is very brittle due to the high degree of hardness, and tends to flake, measures have been disclosed for removing the connection zone. DE 3506746 C2 discloses a generic cast-iron or steel piston ring in which the connection layer was removed so the diffusion layer can be sprayed or coated with a different layer. Because the removal of the connection layer constitutes an additional work cycle in the assembly line of the piston-ring manufacture, the overall manufacturing costs are relatively high.
It is the object of the invention to provide a low-wear piston ring with advantageously-low manufacturing costs.